It is known to scrub acid gases, especially carbon dioxide, hydrogen sulfide or other compounds such as carbon-oxy sulfides, with aqueous solutions of amines.
In German patent document No. 19 517 51, for example, a number of different compounds from the amine class have been described for this purpose. The amines are used in an aqueous solution for the scrubbing of such acid components from pyrolysis (cracking) gases.
Amont the amines used for this purpose are monoethanol amine and ethylene diamine.
The use of such scrubbing liquids is not limited to the cleaning of pyrolysis gases but can also be applied in a number of other technological fields and to gas mixtures generated therein, especially coal-gasification gases and synthesis gases.
In the selection of a scrubbing liquid for acid gases, various parameters must be considered. For example, each amine solution has a maximum absorption capacity, absorption velocity, regeneratability and corrosivity. Furthermore, one must also consider the selectivity with respect to the components to be scrubbed from the gas mixture and the chemical stability of the active components.
In each process, therefore, within the boundary conditions, there is generally a compromise between and among these various parameters.
None of the known scrubbing liquids is optimal for all of these parameters and in all of these aspects. As a result, the selection of the active agent in this scrubbing liquid is usually as a result of a compromise or a choice depending upon which of the parameters are most important to optimize. Generally the active agent of the scrubbing liquid is selected based upon tests for the particular gas mixture concerned and the environment in which the scrubbing is to occur.
A particularly significant problem in the scrubbing of acid gases from gas mixtures containing same is the problem of corrosion of the apparatus which generally consists of steel.
A reduction in the corrosivity of the scrubbing liquid can decrease the cost of the apparatus by avoiding the need for expensive corrosion-resistant steels and reduce the replacement costs by increasing the apparatus life.
In Chemical Engineering Progress, volume 69, No. 2, February 1973, pp 57-61, a number of investigations are described for inhibiting the corrosion of the system monoethanol amine-water-carbon dioxide-steel. In this publication, a three-component inhibitor system referred to as "amine guard" is found to be the best. Because such corrosion inhibitors permit ordinary steels to be used in place of corrosion-resistant steels, their use does not pose an insurmountable cost factor. However, a constant chemical control of the composition of the scrubbing liquid is required when inhibitors are used and losses of the inhibiting components must be continuously monitored and replaced.
Another important consideration in such scrubbing liquids is the absorptive capacity of the active ingredient of the scrubbing liquid since an increased absorptive capacity means that a reduced volume of scrubbing liquid can be employed. This reduction in the throughput of the scrubbing liquid can allow a reduction in the associated apparatus components, especially the components of the scrubbing-liquid circulating path, for example, the heat exchangers, and a reduction in the pumping energy. Both the operating and capital cost of the system can therefore be reduced.